Pure-blood is the term for wizards and witches who come from "purely" magical heritage. They claim to have no Muggles in their family trees. Because the only way for a family to retain their pure-blood status is to marry other pure-bloods, all pure-blood families were related to one another by the 1990s. Because of inbreeding among pure-bloods and some marrying Muggles and Muggle-borns, the number of families claiming to be pure-blood is declining[1].

At the very least, a witch or wizard must have all magical grandparents in order to be considered a pure-blood. However, the definitions of blood status were invented by those who are prejudiced against non-purebloods. They consider Muggle-borns to be as "bad" as Muggles[2], and therefore, it is likely that a wizard or witch must not have any Muggle-born grandparents either in order to be considered pure-blood.

Some pure-blood families can trace their pure-blood status through many generations of magical ancestors and deny ever having any Muggles within the family, such as the House of Black. However, the truth is that if they ever did exist in the past, true pure-blood wizards and witches do not exist today. They merely erase Squibs,Muggle-borns and Muggles from their family trees.[1]

Those who consider blood purity very important also hide their Muggle ancestry, clinging to the magical heritage they do have[3]. Many Death Eaters are believed to have done this, and their leader himself took on the name Lord Voldemort out of a desire not to keep the name of his "filthy Muggle father"[4]. It is likely that Voldemort told his followers he was a pure-blood, given the reaction of Bellatrix Lestrange to Harry's "accusation" that he was in fact a half-blood[5].

Many pure-blood families, such as the Blacks and Gaunts, practice marrying cousins in order to maintain their pure-blood status, and disown any members who marry someone who is not a pure-blood. This practice has been known to cause problems as families become inbred; for example, the Gaunts showed signs of violent tendencies, mental instability, and even diminished magical capability.[3] The number of pure-bloods is diminishing over time because of inbreeding[1].

It is theoretically possible for a family to "regain" pure-blood status after a time. For example, although Harry Potter's mother Lily was Muggle-born, thereby making Harry a half-blood, his wife Ginny was a pure-blood. If each of their three children were to marry pure-bloods, any children they had could be considered "pure-blood" as well, as they would have no Muggles or Muggle-borns among their grandparents.

"I really don't think they should let the other sort in, do you? They’re just not the same, they've never been brought up to know our ways. Some of them have never even heard of Hogwarts until they get the letter, imagine. I think they should keep it in the old wizarding families."

Some pure-bloods regard themselves as superior to those witches and wizards who are born to Muggles and half-bloods. They believe that Muggles are little more than animals, and that Muggle-borns, whom they derisively call "Mudbloods", are second-class citizens who are inferior and unworthy of being allowed to practice magic. Many also discriminate against half-breeds, werewolves, and magical creatures such as Centaurs.

However, there are pure-bloods who do not have these beliefs, and instead treat all magical people and Muggles as equals. These pure-blood folk are often labelled "blood traitors" by the more fanatical pure-bloods.[4]

The expressions ‘pure-blood’, ‘half-blood’ and ‘Muggle-born’ have been coined by people to whom these distinctions matter, and express their originators’ prejudice. As far as somebody like Lucius Malfoy is concerned, for instance, a Muggle-born is as ‘bad’ as a Muggle. Therefore Harry would be considered only ‘half’ wizard, because of his maternal grandparents. If you think this is far-fetched, look at some of the real charts the Nazis used to show what constituted ‘Aryan’ or ‘Jewish’ blood...the Nazis used precisely the same warped logic as the Death Eaters. A single Jewish grandparent 'polluted' the blood, according to their propaganda.[2]

This statement may indicate that all of a wizard or witch's grandparents must have magical heritage — i.e. that he or she cannot have a single Muggle or Muggle-born grandparent — in order for him or her to be considered pure-blood. The detailed and lengthy family trees maintained by pure-blood families such as the House of Black[5] suggests that more than two generations of magical heritage are required to be considered pure-blood. However, this may only be the standard applied by extremists.

On J. K. Rowling's draft list of students in Harry Potter's class, which is not considered canon due to several contradictions with the novels, the following students were mentioned as pure-bloods[21]:

↑Ch. 13 of Deathly Hallows confirms that the Selwyns are a pure-blood family, though it is unclear if Umbridge was being truthful about her own relation to them or her own blood status, as she was trying to "bolster her own pure-blood credentials". See Talk:Selwyn family for more information.

↑Slytherin’s only known descendants died with the Gaunts and Tom Riddle.

↑Bilius is a relative of the Weasleys, though it is unknown if he is a relation of Arthur or Molly.